ASP.NET: DropDownList do not fire onselectdindexchanged

I add a DropDownList in a web page and the setting is shown below:

<asp:DropDownList ID="DropDownList1" runat="server" Height="35px"
            onselectedindexchanged="DropDownList1_SelectedIndexChanged" Width="184px" 
            style="margin-right: 38px">

When I change the value in the DropDownList, it do not trigger the onselectedindexchange method. After some searching, i need to add attribute AutoPostBack

AutoPostBack="true"